The exhaust gas discharged from a combustion chamber contains unburned gas, NOX, etc. To remove such components of the exhaust gas, an exhaust purification catalyst is arranged in an engine exhaust passage. As an exhaust purification catalyst able to simultaneously remove unburned gas, NOX, and other components, a three-way catalyst is known. A three-way catalyst can remove unburned gas, NOX, etc. with a high removal rate when an air-fuel ratio of the exhaust gas is near a stoichiometric air-fuel ratio. For this reason, there is known a control system which provides an air-fuel ratio sensor in an exhaust passage of an internal combustion engine and uses the output value of this air-fuel ratio sensor as the basis to control an amount of fuel fed to the internal combustion engine.
As the exhaust purification catalyst, one having an oxygen storage ability can be used. An exhaust purification catalyst having an oxygen storage ability can remove unburned gas (HC, CO, etc.), NOX, etc. when the oxygen storage amount is a suitable amount between an upper limit storage amount and a lower limit storage amount even_if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is rich. If exhaust gas of an air-fuel ratio at the rich side from the stoichiometric air-fuel ratio (below, referred to as a “rich air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen stored in the exhaust purification catalyst is used to remove by oxidation the unburned gas in the exhaust gas.
Conversely, if exhaust gas of an air-fuel ratio at a lean side from the stoichiometric air-fuel ratio (below, referred to as a “lean air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen in the exhaust gas is stored in the exhaust purification catalyst. Due to this, the surface of the exhaust purification catalyst becomes an oxygen deficient state. Along with this, the NOX in the exhaust gas is removed by reduction. In this way, the exhaust purification catalyst can purify the exhaust gas so long as the oxygen storage amount is a suitable amount regardless of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst.
Therefore, in such a control system, to maintain the oxygen storage amount at the exhaust purification catalyst at a suitable amount, an air-fuel ratio sensor is provided at the upstream side of the exhaust purification catalyst in the direction of flow of exhaust, and an oxygen sensor is provided at the downstream side in the direction of flow of exhaust. Using these sensors, the control system uses the output of the upstream side air-fuel ratio sensor as the basis for feedback control so that the output of this air-fuel ratio sensor becomes a target value corresponding to the target air-fuel ratio. In addition, the output of the downstream side oxygen sensor is used as the basis to correct the target value of the upstream side air-fuel ratio sensor.
For example, in the control system described in Japanese Patent Publication No. 2011-069337A, when the output voltage of the downstream side oxygen sensor is a high side threshold value or more and the exhaust purification catalyst is in an oxygen deficient state, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made a lean air-fuel ratio. Conversely, when the output voltage of the downstream side oxygen sensor is a low side threshold value or less and the exhaust purification catalyst is in an oxygen excess state, the target air-fuel ratio is made a rich air-fuel ratio. Due to this control, when in the oxygen deficient state or oxygen excess state, it is considered possible to quickly return the state of the exhaust purification catalyst to a state between these two states, that is, a state where the exhaust purification catalyst stores a suitable amount of oxygen.
Further, in the control system described in Japanese Patent Publication No. 2001-234787A, the outputs of an air flowmeter and upstream side air-fuel ratio sensor of an exhaust purification catalyst etc. are used as the basis to calculate an oxygen storage amount of the exhaust purification catalyst. In addition, when the calculated oxygen storage amount is larger than a target oxygen storage amount, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made a rich air-fuel ratio, and when the calculated oxygen storage amount is smaller than a target oxygen storage amount, the target air-fuel ratio is made the lean air-fuel ratio. Due to this control, it is considered that the oxygen storage amount of the exhaust purification catalyst can be maintained constant at the target oxygen storage amount.